There's a number of things you really need to consider with both sensor types - range, direction and speed of operation.
A lot of the simple on/off outputs are quicker to operate than the a ranging sensor with a variable output. Considering the speed of opponents you might disadvantage yourself if you're turning to where the opponent was a short time ago. Unless you're scanning the sensor over the area you may need more that one sensor to determine opponent location... in front, to the left or right?
Some sensors have a dead zone up close, can you manage that? Some IR sensors are short range, will that hinder your detection of the the opponent? If you're using longer range IR or ultrasonic will you be picking up on objects right out of the area?
There's a little more thought on this here: minisumo.org.uk/wiki/OpponentDetection